1. Field of the Invention
The present invention relates to photoelectrically controlled switching devices, and, more particularly, to a device for automatically starting and stopping a gas burner, especially a bunsen burner, in response to shading from ambient light of a photoelectric control sensor, due, for example, to the approach of a hand.
2. Description of the Prior Art
U.S. Pat. No. 4,028,047 discloses a bunsen burner with photoelectrically operated start-stop controls of the type mentioned above. This apparatus features a box-like housing with a flexible hose leading to a gas supply and an electrical cord for plug-in into an electrical wall outlet. A bunsen burner mounted on top of the housing is connected to the gas supply line via a solenoid-controlled shutoff valve. The solenoid is operated by a relay and a dimmer switch circuit which consists primarily of a photoelectric cell and an amplification stage.
The photoelectric cell is normally so adjusted that, under the incidence of ambient light, the solenoid valve remains closed and the bunsen burner is shut down. A directionally adjustable light tube above the photoelectric cell is oriented in such a way that the presence of an implement holding hand in the vicinity of the bunsen burner will reduce the incidence of light at the photoelectric cell by throwing a shadow over it, thereby triggering the solenoid valve to open the gas supply line. The burner flame is ignited by means of an electrical glow igniter or some other suitable ignition means. As soon as the operator removes his hand from the vicinity of the bunsen burner, the shadow disappears from the light tube and the dimmer switch circuit automatically shuts down the gas supply.
The advantages of this type of automatic burner operation are numerous. By completely eliminating the need for manually opening and closing the burner valve, it frees both hands of the operator. This feature is particularly useful in situations, where frequent short usage of a bunsen burner is common, as in the case in dental labs, for example. It is the current practice for a dental technician, rather than manually opening and closing the burner valve repeatedly, to simply leave the burner on, even when not needing it. The use of an automatically operating gas valve thus reduces to a minimum the time during which the gas flame is burning, thereby simultaneously minimizing the deterioration of the air in the lab, the consumption of energy, and the undesirable heating action of the burner on a hot day.
It has been found, however, that this prior art apparatus is subject to malfunction, under certain circumstances. This is particularly the case, when the ambient light in the area in which the burner is located is subject to fluctuations which can trigger the unintentional opening or closing of the solenoid valve, such as, when sunlight suddenly falls on the apparatus, or suddenly ceases to fall on the apparatus, or when interior electrical light sources are switched on or off.